The present invention relates to a method and an apparatus for blanking out interfering noise in connection with detecting knocking in an internal combustion engine.
Knocking is known to cause abnormal interference with combustion in internal combustion engines and causes mechanical pressure waves or high-frequency structure-borne sound, which over long-term operation can cause severe engine damage.
The tendency to knocking can be reduced by various provisions, among others by means of short combustion paths with a central spark plug location, a compact combustion chamber, high turbulence in the combustion chamber, higher-octane fuel, the avoidance of hot spots in the combustion chamber, a lower compression ratio, and a colder mixture intake temperature, and so forth.
Since for good fuel consumption the engine should always be operated near the knocking limit, electronic engine controllers typically have a knocking controller. An important ingredient of this knocking controller is a knocking sensor, which detects the high-frequency oscillations of knocking at the cylinder wall and converts them into electrical oscillations, which are then analyzed for the presence of knocking.
For this purpose, known knocking sensors are connected to a knocking sensor evaluation IC via an input circuit. In the knocking sensor evaluation IC, the high-frequency sensor signal is amplified, filtered, and, during an observation period (measurement time slot), integrated. The outcome of the integrator is read in and processed by a microcontroller and used for knocking detection. From the processed integrator signal, a sliding mean value (reference level) is determined, on the basis of which the occurrence of knocking can be ascertained.
However, knocking sensors detect not only the structure-borne sound originating from combustion but also structure-borne sound interfering noises. For instance, in direct-injection 3- and 6-cylinder engines, overlaps can occur between the measurement time slot for knocking and the onset and end of injection, that is, the actuation in each case of the injection valve.
The noises that the injection valve causes when it opens and closes are transmitted to the engine housing and become perceptible in the structure-borne sound picked up by the knocking sensor.
If these interfering noises migrate into the measurement time slot, they can be erroneously interpreted by the knocking controller as knocking. This leads to unnecessary retardation of the ignition angle and thus an unnecessarily reduced engine torque.
If the opening and closing of the injection valve is always within the measurement time slot for the knocking, this raises the reference level of knocking detection, making the knocking detection poorer. The result of this can be that knocking cannot be detected, resulting in engine damage.
Besides the 3- and 6-cylinder engines already mentioned, the problem of overlapping of the injection and the measurement time slot for the knocking can also occur in future in multiple-injection engines with any other number of cylinders.
FIG. 4, in further detail, illustrates the problems on which the present invention is based, in terms of the course over time of both the knocking control measurement time slot and the interference time slot, the latter being caused for instance by the opening and closing of the injection valve.
In FIG. 4, MF stands for the measurement time slot, EF stands for the interference time slot, t1 through t4 are times, and t is time in general.
In the illustration in FIG. 4, the measurement time slot lasts from a first time t1 to a second time t2. Within this time period, the output signal of the knocking sensor is integrated in a known manner, so as to obtain a sensor signal integral value. Typically, the measurement time slot MF is a few milliseconds long.
The interference time slot EF lasts from time t3 to time t4 and is located entirely inside the measurement time slot MF. The structure-borne sound additionally detected in the interference time slot EF thus adulterates the sensor signal integral value ascertained.
FIG. 5 shows a typical course of intensity over time I(t) of the interference signal inside the interference time slot EF; this signal originates for instance in the opening and closing of the injection valve.
As can be seen from FIG. 5, the interference typically lasts 1 ms, and the course over time initially has a rapid rise to a maximum value Im, followed by an exponential drop to 0. Reference symbols IW and IW′ designate the integral values of the course of intensity over time between 0 and 0.5 ms and between 0.5 and 1 ms, respectively. As FIG. 5 clearly shows, the majority of the integral intensity, namely IW′, is in the range between 0 and 0.5 ms, that is, at the onset of the opening or closing of the injection valve.